1. Field of the Invention
This invention relates generally to telecommunication systems and assemblies, and more particularly to an AOTF and a system that produces longitudinal acoustic waves used to couple between modes propagating in the fiber.
2. Description of Related Art
In modern telecommunication systems, many operations with digital signals are performed on an optical layer. For example, digital signals are optically amplified, multiplexed and demultiplexed. In long fiber transmission lines, the amplification function is performed by Erbium Doped Fiber Amplifiers (EDFA""s). The amplifier is able to compensate for power loss related to signal absorption, but it is unable to correct the signal distortion caused by linear dispersion, 4-wave mixing, polarization distortion and other propagation effects, and to get rid of noise accumulation along the transmission line. For these reasons, after the cascade of multiple amplifiers the optical signal has to be regenerated every few hundred kilometers. In practice, the regeneration is performed with electronic repeaters using optical-to-electronic conversion. However to decrease system cost and improve its reliability it is desirable to develop a system and a method of regeneration, or signal refreshing, without optical to electronic conversion. An optical repeater that amplifies and reshapes an input pulse without converting the pulse into the electrical domain is disclosed, for example, in the U.S. Pat. No. 4,971,417, xe2x80x9cRadiation-Hardened Optical Repeaterxe2x80x9d. The repeater comprises an optical gain device and an optical thresholding material producing the output signal when the intensity of the signal exceeds a threshold. The optical thresholding material such as polydiacetylene thereby performs a pulse shaping function. The nonlinear parameters of polydiacetylene are still under investigation, and its ability to function in an optically thresholding device has to be confirmed.
Another function vital to the telecommunication systems currently performed electronically is signal switching. The switching function is next to be performed on the optical level, especially in the Wavelength Division Multiplexing (WDM) systems. There are two types of optical switches currently under consideration. First, there are wavelength insensitive fiber-to-fiber switches. These switches (mechanical, thermo and electro-optical etc.) are dedicated to redirect the traffic from one optical fiber to another, and will be primarily used for network restoration and reconfiguration. For these purposes, the switching time of about 1 msec (typical for most of these switches) is adequate; however the existing switches do not satisfy the requirements for low cost, reliability and low insertion loss. Second, there are wavelength sensitive switches for WDM systems. In dense WDM systems having a small channel separation, the optical switching is seen as a wavelength sensitive procedure. A small fraction of the traffic carried by specific wavelength should be dropped and added at the intermediate communication node, with the rest of the traffic redirected to different fibers without optical to electronic conversion. This functionality promises significant cost saving in the future networks. Existing wavelength sensitive optical switches are usually bulky, power-consuming and introduce significant loss related to fiber-to-chip mode conversion. Mechanical switches interrupt the traffic stream during the switching time. Acousto-optic tunable filters, made in bulk optic or integrated optic forms, (AOTFs) where the WDM channels are split off by coherent interaction of the acoustic and optical fields though fast, less than about 1 microsecond, are polarization and temperature dependent. Furthermore, the best AOTF consumes several watts of RF power, has spectral resolution about 3 nm between the adjacent channels (which is not adequate for current WDM requirements), and introduces over 5 dB loss because of fiber-to-chip mode conversions.
Another wavelength-sensitive optical switch may be implemented with a tunable Fabry Perot filter (TFPF). When the filter is aligned to a specific wavelength, it is transparent to the incoming optical power. Though the filter mirrors are almost 100% reflective no power is reflected back from the filter. With the wavelength changed or the filter detuned (for example, by tilting the back mirror), the filter becomes almost totally reflective. With the optical circulator in front of the filter, the reflected power may be redirected from the incident port. The most advanced TFPF with mirrors built into the fiber and PZT alignment actuators have only 0.8 dB loss. The disadvantage of these filters is a need for active feedback and a reference element for frequency stability.
The use of conventional optical fibers has presented difficulties in mode coupling between the fundamental and symmetric higher-order optical modes. PDL is a problem using asymmetric modes with transverse acoustic waves. One solution is to use longitudinal acoustic waves. However, there is not much overlap using a longitudinal wave this makes the symmetric to symmetric mode coupling difficult. There is a need to use longitudinal acoustic waves. However, efficiency problems pose problems.
The mode coupling coefficient between the fundamental core mode and a higher-order symmetric mode by the acoustic wave is proportional to the overlap integral between the electric fields of the two modes multiplied by the acoustically-induced refractive-index change, i.e.,
xcexaxe2x88x9d∫E1(r,xcfx86)E2(r,xcfx86)xcex4n(r,xcfx86)drdxcfx86
At a high acoustic frequency, xcexa=0 since xcex4n is almost zero at the region where E1 and E2
xcexaxe2x88x9dxcex4n∫E1(r,xcfx86)E2(r,xcfx86)drdxcfx86
overlap, but even at a low frequency, xcexa is small since xcex4n=constant and from the mode orthogonality,
For this reason, an acousto-optic (AO) device using a conventional optical fiber with the lowest-order longitudinal acoustic wave is difficult.
There is a need for an AO device that uses a higher acoustic wave. There is a need for an AO device that generates a longitudinal acoustic wave which is used to couple between modes propagating in the fiber.
Accordingly, an object of the invention is to provide AO devices that use higher acoustic waves.
Another object of the present invention is to provide AO devices that use longitudinal acoustical waves to couple between modes propagating in the fiber.
Yet another object of the present invention is to provide a tunable AOTF that uses longitudinal acoustical waves to couple between modes propagating in the fiber.
These and other objects of the present invention are achieved in an acousto-optic filter. The filter includes an optical fiber with a longitudinal axis, a core and a cladding in a surrounding relationship to the core. The optical fiber has multiple cladding modes. At least one acoustic wave generator made of a sufficiently high frequency piezoelectric material to produce a longitudinal acoustic wave. An acoustic wave propagation member has proximal and distal ends. The distal end is coupled to the optical fiber and the proximal end is coupled to the acoustic wave generator. The acoustic wave propagation member propagates the longitudinal acoustic wave from the proximal to the distal ends and excites the longitudinal acoustic wave in the optical fiber.